It makes no difference what the objects of investigation are. If it authorizes warrantless searches, it's unconstitutional."We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves..."
Your point? Citizens can still be caught up in investigations of foreign operations if someone decides they're connected in some way.